In a solar cell comprising crystalline semiconductor material especially, a crystalline silicon solar cell comprising monocrystalline silicon or polycrystalline silicon, it is desirable, that a semiconductor silicon layer serving as a power generating layer be thinner than 100 .mu.m to reduce the production cost and improve the Conversion efficiency. Because of the reduction in the thickness of the power generating layer, the quantity of silicon semiconductor required in the production of the solar cell is reduced, whereby the material cost is reduced. Further, although a conventional solar cell comprising a crystalline silicon wafer is about 500 .mu.m thick, actually 100 .mu.m is enough to absorb solar light. In order to confine incident light in the power generating layer to provide an adequate optical path for long wavelength light, i.e., in order to perform so-called "optical confinement" with high efficiency, the power generating layer is desired to be thinner than 100 .mu.m. In a power generating layer thinner than 100 .mu.m, the minority carriers produced due to incident light are collected with high efficiency, resulting in a solar cell with excellent conversion efficiency.
For the reasons described above, in various institutes, attempts have been made to employ thin semiconductor films, especially, thin crystalline silicon films, in the production of solar cells. Since a thin semiconductor film has poor mechanical strength, usually it is formed on a support substrate. The support substrate must have a heat-resisting property because a high temperature process is employed in the production of the thin semiconductor film. Furthermore, since semiconductors are structure sensitive, addition of impurities in a very small amount causes generation of levels in the forbidden band, whereby electric characteristics vary, resulting in a reduction in the efficiency of the solar cell. Therefore, the support substrate must not-be a supply source of the adverse impurities during the high temperature process. Accordingly, materials satisfying the above-described severe conditions are restricted, and such specific materials are not cheap, so that it is against the purpose of reducing the total cost of the solar cell to employ such specific materials.
Furthermore, techniques of forming the thin semiconductor films have various difficulties. Generally, when a thin semiconductor film is formed on a support substrate comprising a material different from the semiconductor film or coated with that material, a semiconductor film that becomes an amorphous film at a relatively low temperature and becomes a polycrystalline film at a relatively high temperature may be employed as the thin semiconductor film. In this case, however, problems with respect to performance and reliability of completed solar cells remain unsolved. Therefore, in this description, the subject is narrowed to crystalline semiconductor films.
The possibility of growth of a polycrystalline semiconductor film, especially a polycrystalline silicon film, on a material of different kind and the properties of the grown semiconductor film depend on the technique and temperature of the growth process. For example, when CVD (Chemical Vapor Deposition) is employed, a thin polycrystalline silicon film can be grown on a material of any kind. In this case, the grain size of the polycrystalline silicon increases as the growth temperature increases. When performance of the solar cell is considered, it is desirable that the grain size be large. However, the grain size of the polycrystalline semiconductor film grown by CVD is only several microns and, in polycrystalline silicon, this Grain size is not enough to realize a solar cell with high conversion efficiency.
There is an idea that a nucleus for the crystal growth can be produced on the support substrate in advance of the growth of the semiconductor film to increase the grain size. However, this idea is contradictory-to the reduction in the cost of a solar cell. For example there is a technique in which a support substrate comprising a semiconductor material is coated with a material-different from the semiconductor material and an aperture is formed through that coating to expose a part of the substrate that is used as a nucleus for the crystal growth. However, since the support substrate comprises a semiconductor material, this technique is incompatible with the idea of reducing the cost of the solar cell by employing a low-priced material for the support substrate. If a low-purity semiconductor material is used for the support substrate to reduce the material cost, impurities contained in that material adversely affect the performance of the solar cell.
Alternatively, there is a technique called "graphoepitaxy" in which a support substrate comprising a material different from a semiconductor material to be grown on the substrate is employed and a pattern of alternating recesses and projections that triggers the regularity of the grown crystal is produced at the surface of the substrate. In this technique, however, the patterning of alternating recesses and projections increases the production costs.
Furthermore, a solar cell including a thin semiconductor film produced on a support substrate has the following drawbacks. That is, in order to output the current generated in the thin semiconductor film due to incident light, an electrode in ohmic contact with the semiconductor film must be interposed between the semiconductor film and the support substrate. Usually, metal is employed as the material of the electrode. However, if a metal electrode is in direct contact with the semiconductor film during high temperature processing, the metal unfavorably diffuses into the semiconductor film, adversely affecting the characteristics of the semiconductor film. Therefore, it is necessary to output the current through the support substance without using such a metal electrode. That is, the support substrate must have the following characteristics:
1) To support the thin semiconductor film. PA1 2) To make an ohmic contact with the thin semiconductor film. PA1 3) To output the current generated in the thin semiconductor film. PA1 4) Not to supply impurities that adversely affect the characteristics of the thin semiconductor film.
However, in practice, a material satisfying all of these conditions does not exist. This result is attributed to the fact that the support substrate should serve both as a substrate for the formation of the thin semiconductor film and as a back electrode of the solar cell.
In order to fabricate a solar cell economically, after a thin semiconductor film is produced on a support substrate that provides an adequate quality semiconductor film, the support substrate is separated from the semiconductor film and reused. Some specific techniques employing reusable substrates in the production of solar cells have been proposed.
FIGS. 31(a)-31(d) are sectional views illustrating process steps in a prior art method for fabricating a thin film solar cell. In these figures, reference numeral 1 designates a heat-resistant substrate comprising silicon, numeral 3 designates a thin semiconductor film comprising p type silicon, numeral 2 designates an intermediate film, such as a silicon oxide film or a calcium fluoride film, to separate the thin semiconductor film 3 from the substrate 1, numeral 5 designates a junction layer, numeral 6 designates grid electrodes, and numeral 7 designates a back electrode.
Initially, as illustrated in FIG. 31(a), a silicon oxide film 2 or the like is deposited on the surface of the heat resistant substrate 1 by CVD or the like and, thereafter, a p type semiconductor film 3 is deposited on the film 2 by CVD or the like.
In the step of FIG. 31(b), a junction layer 5 is formed by diffusing n type impurities from the surface of the thin semiconductor film 3 or forming an n type microcrystalline film on semiconductor film 3, producing a pn junction.
Thereafter, the semiconductor film 3 is released from the substrate 1 by etching the intermediate film, i.e., the silicon oxide film, 2 with hydrofluoric acid as an etchant, and peeling the semiconductor film 3 from the substrate 1 (FIG. 31(c)). When the intermediate film 2 comprises calcium fluoride, aqueous ammonia is used as the etchant.
Finally, as illustrated in FIG. 31(d), grid electrodes 6 and a back electrode 7 are formed on the junction layer 5 and the rear surface of the semiconductor film 3, respectively, by sputtering of metal, completing a thin film solar cell.
In this production method, if the silicon oxide film 2 is several microns thick and the thin semiconductor film 3 is 10 square centimeters in size, it takes a long time, exceeding 1000 hours, to completely etch the silicon oxide film 2. Therefore, this process is not fit for practical use. When a calcium fluoride film is employed, calcium or other impurity elements contained in that film are unfavorably mixed into the semiconductor film 3, whereby the quality of the semiconductor film 3 is degraded resulting in a thin film solar cell with poor performance.
FIGS. 32(a)-32(e) are sectional views illustrating process steps in a prior art method for fabricating a thin film solar cell disclosed in Japanese Published Patent Application No. Hei. 4-333288. In these figures, reference numeral 1 designates a heat-resistant substrate comprising a silicon wafer, numeral 11 designates an insulating film pattern, such as a silicon oxide film, numeral 12 designates a first silicon layer having a relatively small resistivity, numeral 13 designates a second silicon film having a relatively large resistivity, numeral 14 designates grooves opposite the insulating film pattern 11, numeral 6 designates grid electrodes, and numeral 7 designates a back electrode.
Initially, as illustrated in FIG. 32(a), a pattern of insulating film 11 is formed on the heat-resistant substrate 1. Thereafter a first silicon layer 12 and a second silicon layer 13 are successively epitaxially grown on the substrate 1 where the insulating-film pattern 11 is absent (FIG. 32(b)). Since no silicon layer is grown on the insulating films 11, a plurality of grooves 14 are formed opposite the insulating films 11. Through these grooves 14, the insulating films 11 are etched away with hydrofluoric acid (FIG. 32(c)). Thereafter, as illustrated by arrows, the first silicon layer 12 is selectively etched away through the grooves 14, with a mixture of hydrofluoric acid, nitric acid, and acetic acid, utilizing a difference in etching rates between the first silicon layer 12 and the second silicon layer 13 due to the difference in the resistivities, whereby the second silicon layer 13 is separated from the substrate 1 (FIG. 32(d)). The process of forming a junction layer 5, grid electrodes 6, and a back electrode 7 as shown in FIG. 32(e) is identical to that already described with respect to FIG. 31(d).
In this prior art method, the first silicon layer 12 to be etched away comprises the same material as the heat-resistant substrate 1 and the second silicon layer 13, and the first silicon layer 12 is selectively etched away utilizing the difference in etching rates between the first silicon layer 12 and the second silicon layer 13 due to the difference in resistivities between these layers. Therefore, the resistivity of the second silicon layer 13 is restricted within a narrow range. In addition, the etchant for the first silicon layer 12 is restricted. In this case it is difficult, to obtain excellent characteristics both in the selectivity of the etching and the etching rate. Therefore, although the separation of the second silicon layer 13 is theoretically possible in this prior art method, this method is not fit for practical use. Further, since the first and second silicon layer 12 and 13 and the substrate 1 comprise the same material, it is practically difficult to remove only the first silicon layer 12 without damaging the second silicon layer 13 and the substrate 1. Although the substrate 1 is reusable, it is damaged in each use, so that the practical lifetime of the substrate is limited. As a result, a practical and low-cost production method of thin film solar cells is not achieved.
Furthermore, this prior art method requires a selective epitaxial growth process for the thin semiconductor films, so that the material and the technique are restricted. In addition it is very difficult to reduce the production costs without degrading the quality of the epitaxially grown layer while improving productivity by increasing the growth rate. Accordingly, this prior art method provides poor efficiency and is not applicable to semiconductor devices that require mass production at low cost, such as solar cells.
FIG. 33 is a sectional view for explaining a prior art method of fabricating a thin film solar cell using a graphite sheet, disclosed in Japanese Published Patent Application No. Hei. 4-186083. In the figure, the same reference numerals as in FIGS. 31(a)-31(d) designate the same or corresponding parts. A graphite sheet is employed as the intermediate layer 2 to separate the thin film 3 from the heat-resisting substrate 1.
In this prior art method, in the step of separating the thin semiconductor film 3 from the substrate 1, a mechanical stress is applied to the graphite sheet 2 to cleave the graphite sheet. Since the separation of the semiconductor film 3 and the substrate 1 is mechanically carried out, if the semiconductor film 3 is as thin as several tens of microns and the mechanical strength of that semiconductor film is poor, the semiconductor film 3 unfavorably breaks due to the stress applied during the separation process. Therefore, in this prior art method, it is difficult to produce thin film solar cells at good yield.
U.S. Pat. No. 4,816,420 also discloses a technique of mechanically separating a thin semiconductor film from a substrate. However, this prior art technique provides the above-described problems unless the materials of the thin semiconductor film, the intermediate layer, and the substrate are appropriately selected so that the adhesion of the intermediate layer to the semiconductor film and the substrate is reduced. For example, when the material of the thin semiconductor film is silicon, such as polycrystalline silicon, it is difficult to select appropriate materials satisfying the above-described condition.
The above-described prior art techniques for separating a thin semiconductor film from a support substrate and reusing the support substrate to reduce the production cost have the following drawbacks. That is, the separation of the thin semiconductor film from the support substrate takes a long time. In addition, it is very difficult to separate the semiconductor film from the support substrate without damaging the film or the substrate. Accordingly, a practical process that produces thin film solar cells at low cost is not proposed yet.
Japanese Published Patent Application No. Sho. 59-161883 discloses a technique of etching a thin optical semiconductor film. In this prior art, a photoresist layer 8 formed on an optical semiconductor film 4 is exposed to ultraviolet light through a mask 12 and a transparent base film 7. Since the adhesion between the photoresist layer 8 and the base film 7 is decreased at portions exposed to the ultraviolet light, when the base film 7 is removed from the photoresist layer 8, portions 8a to 8c of the photoresist layer are left on the optical semiconductor film 4. Then, the optical semiconductor film 4 is selectively etched using the photoresist layers 8a to 8c as masks. This prior art relates to a technique for partially removing the photoresist layer to make a plurality of holes in that photoresist layer through which the underlying semiconductor film is selectively etched. Therefore, this technique is not for separating a thin semiconductor film for a solar cell from a substrate.
Japanese Published Patent Application No. Hei. 3-250771 discloses a method of producing a photovoltaic device. In this method, a first transparent electrode 2, a semiconductor optical active layer 3, and a second transparent electrode 4 are successively formed on a substrate 1, and separation grooves 5, 6, and 9 having prescribed depths are formed through the structure, followed by deposition of an electrode layer 10, whereby the structure is divided into a plurality of photovoltaic elements and these photovoltaic elements are connected in series. This prior art relates to a technique for electrically dividing the semiconductor active layer on the substrate with grooves reaching the substrate, but this technique is not for separating the semiconductor active layer from the substrate.
Japanese Published Patent Application No. Hei. 4-296061 discloses a method of producing a photovoltaic device including forming a flexible photovoltaic element on a substrate via a mold release layer, cutting the mold release layer and the photovoltaic element in an island shape, and immersing the substrate in water to separate the photovoltaic element from the substrate at the mold release layer. In this prior art, however, it is premised that a resin layer is interposed between the substrate and a power generating layer. In addition, a portion of the photovoltaic element separated from the substrate is smaller than the photovoltaic element produced on the substrate. Further, a groove determining the portion to be separated must be formed in advance of the cutting.
FIG. 34 is a perspective view illustrating a prior art solar cell disclosed in Japanese Published Patent Application No. Hei. 2-51282. FIG. 35 is a sectional view of a part of the solar cell. In these figures, reference numeral 21 designates a p type semiconductor substrate, numeral 22 designates an n type semiconductor layer, numeral 26 designates an anti-reflection-film, numerals 36 and 37 designate metal electrode layers, numeral 38 designates through-holes, numeral 39 designates a p.sup.+ type semiconductor layer, and numeral 40 designates a passivation film.
In this solar cell, the n type semiconductor layer 22 is disposed at the surface of the p type semiconductor substrate 21 to produce a pn junction, and the metal electrodes 36 and 37 are in contact with the n type semiconductor layer 22 and the p type semiconductor substrate 21, respectively. Practically, monocrystalline or polycrystalline silicon is employed as a material of the semiconductor substrate 21. The thickness of the substrate 21 depends on the size of the solar cell. In a solar cell of a practical size, i.e., 10 cm.times.10 cm, the substrate 21 is about 500 .mu.m thick. It must be at least as thick as 200 .mu.m because a semiconductor substrate thinner than 200 .mu.m does not have adequate mechanical strength and breaks easily. The p.sup.+ type semiconductor layer 39 improves the electrical ohmic contact between the semiconductor substrate 21 and the metal electrode 36 and produces a high-low junction with the p type semiconductor substrate 21 that provides a BSF (Back Surface Field) effect.
In this solar cell, light is incident on the surface of the n type semiconductor layer 22 disposed on the front surface of the semiconductor substrate 21. The n type semiconductor layer 22 extends to the rear surface of the substrate 21 along the inner wall of the through-hole 38, and the metal electrode 36 is in contact with the n type semiconductor layer 22 at the rear surface of the substrate 21. In this structure, since no metal electrode is present on the light incident surface, the incident light is effectively used.
In this solar cell, however, since current is transferred to the metal electrode 37 through the thin portion of the n type semiconductor layer 22 at the inner wall of the through-hole 38, the electric resistance at that portion causes a loss, reducing the electric power output from the solar cell. The electric resistance increases as the length of the thin portion of then n type semiconductor layer 22 increases, with an increase in the thickness of the semiconductor substrate 21. The electric resistance can be reduced when the diameter of the through-hole 38 is increased and a thick n type semiconductor layer 22 is formed at the inner wall of the through-hole. In this case, however, the light incident area is significantly reduced by the through-holes even through the metal electrodes 36 and 37 are produced on the rear surface of the semiconductor substrate 21.